The present invention relates generally to locating an underground utility conveyance, and more particularly, to a method and apparatus for selectively locating a short-haul cable or pipe that is coupled to a long-haul (backbone) cable.
Many utilities, such as AT&T Corp., bury their pipes and cables (“utility conveyances”) underground both for reasons of safety and esthetics. Underground burial often provides protection to such utility conveyances against weather and other sources of potential damage. Utilities that undertake burial of their conveyances usually make extensive efforts to plot the location of each buried conveyance on a map to facilitate its location in case of repair or replacement. While a map will indicate the general location of a buried conveyance, more precise location information often becomes necessary, particularly in urban environments. For that reason, most utilities that bury their conveyances underground typically rely on electromagnetic signaling techniques to precisely locate such conveyances.
U.S. Pat. No. 5,644,237 to Eslambolchi et al. (incorporated by reference herein) and assigned to the assignee of the present invention, discloses an electromagnetic signaling technique for locating a buried utility conveyance. A signal generator applies a locating signal and a confirmation signal to a metallic part of the conveyance. In the case of an optical fiber cable, the metallic part comprises either a metallic sheath or a copper trace wire within the cable. Using a signal detector, a technician detects both the locating signal and the confirmation signal radiated above ground to precisely locate the buried conveyance.
Conveyance location via electromagnetic signaling works well for long haul conveyances. However, some conveyance routes, and in particular, certain fiber cable routes in urban areas, include one or more short haul cables (“side-leg”) cables that run between a long-haul (backbone) cable and a telecommunications facility. Each side leg cable connects the metallic sheath of the backbone cable to ground, thus allowing the locating signal on the backbone cable to pass into the side-leg cable. All other considerations being equal, a side-leg cable having a smaller impedance (i.e., a shorter signal path to the cable locating signal generator) will radiate a higher strength locating signal. Conversely, the side leg cable having a larger impedance (i.e., a longer signal path to the cable locating signal generator) will radiate a lower strength locating signal, making locating of such a side-leg cable more difficult. In the past, overcoming the problem of a low-strength locating signal on a side-leg cable required the addition of a separate locating signal generator for that side-leg. Such locating signal generators are expensive, and adding a separate locating signal generator for each side-leg will greatly increase costs.
U.S. Pat. No. 6,700,380 to Eslambolchi et al. (incorporated by reference herein) and assigned to the assignee of the present invention, discloses a method for selectively applying a conveyance-locating signal to each of a plurality of short haul (side leg) underground utility conveyances, each coupled to a backbone conveyance. In this expedient, a conveyance-locating signal generator applies a plurality of different frequency conveyance-locating signals to the backbone conveyance. A notch filter, comprising a part of a filter arrestor assembly, connects each short-haul conveyance to ground to selectively pass a particular frequency locating signal. In this manner, each short-haul conveyance carries a locating signal of a particular frequency having a strength that remains unaffected by locating signals on the other short-haul conveyances.
In view of the foregoing, there exists a need for a cable locating system and method that enables selective grounding of the short-haul conveyances to facilitate cable locating procedures without the need for a technician to physically travel to multiple locations to apply and remove the ground.